One aspect of the invention relates to a method and device for determining the oxygen saturation of blood and, in particular, intra-cellular or extra-cellular hemoglobin with spectroscopic analysis. A further aspect of the invention relates to a method of detecting failed autoregulation from spectroscopic analysis of a retina including retinal vessels. Another aspect of the invention relates to a method and device for determining the hematocrit of blood using information derived from spectroscopic analysis and or macular degeneration using information derived from spectroscopic analysis of a retina including retinal vessels.
A variety of spectroscopic oximetry techniques have been proposed for monitoring the blood oxygen saturation and blood oxygen content in retinal vessels. By successfully monitoring the blood oxygen saturation, the arteriovenous oxygen difference can be determined as described by U.S. Pat. No. 5,308,919 to Thomas E. Minnich, U.S. Pat. No. 5,776,060 to Matthew H. Smith, et al., and U.S. Pat. No. 5,935,076 to Matthew H. Smith, et al. Based upon the arteriovenous oxygen difference, the cardiac output of a subject can be approximated in order to assist in post-operative monitoring and the management of critically ill patients. By monitoring the blood oxygen saturation, the loss of blood can be detected, and the rate and quantity of blood loss over time can be estimated as described by U.S. Pat. No. 5,119,814 to Thomas E. Minnich.
In addition to the variety of invasive techniques that require blood to be drawn, oftentimes repeatedly, from a patient, a number of non-invasive spectroscopic oximetry techniques have been proposed and attempted with the intent to measure the blood oxygen saturation of a patient without requiring blood to be drawn from the patient. For example, a number of noninvasive spectroscopic oximetry techniques have been proposed which attempt to measure the blood oxygen saturation of a patient based upon the transmittance of the blood within a retinal vessel, such as a retinal vein or a retinal artery. For example, U.S. Pat. Nos. 5,776,060 and 5,935,076 describe techniques for measuring the oxygen saturation of blood within a retinal vessel by illuminating the retinal vessel with light having a combination of wavelengths and then measuring the transmittance of the blood within the retinal vessel in response to the illumination at each of the selected wavelengths. Based upon the respective transmittance of the blood within the retinal vessel that is measured at each of the selected wavelengths, the oxygen saturation of the blood within the retinal vessel can be approximated. The contents of U.S. Pat. Nos. 5,776,060 and 5,935,076 are hereby incorporated by reference in their entirety.
As will be apparent, the light with which a retinal vessel is illuminated can be reflected and transmitted in a variety of different manners. For example, some of the light will be immediately reflected by the retinal vessel, while other portions of the light will be backscattered by the red blood cells within the retinal vessel. Other portions of the light, termed “double pass light”, will pass through the retinal vessel, be reflected from the retinal and/or choroidal layers and again pass through the retinal vessel, thereby traversing the retinal vessel twice. Further, some portion of the light, termed “single pass light”, will pass through the retinal vessel, diffuse laterally through the retinal and/or choroidal layers and then exit the pupil without again traversing the retinal vessel.
Regardless of the particular paths traveled by the optical signals, the optical signals that return from the eye are collected by a detector and then an associated processing element, such as a microprocessor, a personal computer or the like, which can determine the blood oxygen saturation within the retinal vessel based upon the light that is returned. In order to determine the blood oxygen saturation, techniques have been developed to account for light that has been reflected and/or transmitted in each of the various manners described above. As a result of the variety of different ways in which light can be reflected and/or transmitted, however, the equations that must be solved to determine the blood oxygen saturation within the retinal vessel are quite complicated and may reduce the accuracy with which the blood oxygen saturation can be determined. Further, the equations used require knowledge of a large number of variables, such as hemoglobin concentration within the blood and path length, which must be determined prior to use of the equations.
It is desired to provide a spectroscopic method to non-invasively determine the oxygen saturation of blood in a living organism, where the method is highly calibrated and accurate relative to prior art spectroscopic techniques (fiber optic catheters are calibrated to +−9% Saturation). It is further desired to provide a spectroscopic method that may be used to accurately determine oxygen saturation of blood that requires a minimum of independent variables and does not vary with such factors as pH, red blood cell concentration, hemoglobin concentration and path length of the interrogating light. It is also desired to use data obtained by spectroscopic analysis to determine hematocrit of a sample, and to determine the thickness of one or more retinal layers for the diagnosis of macular degeneration and other retinal diseases.